Total disc replacement involves surgically replacing an intervertebral disc with an artificial implant to treat degenerative disc disease and stenosis on the cervical, thoracic, or lumbar spine.
There are various artificial discs in the prior art. The artificial discs in the prior art could be categorized in three general types: mechanical, composite, and elastic.
Mechanical discs comprise two base plates pivotally coupled by, for example, a ball and socket. An example of a mechanical disc is a product manufactured by MEDTRONIC and sold as “PRESTIGE.” One disadvantage to mechanical discs is that they do not provide sufficient shock absorption. Another disadvantage is that they create a lot of friction and wear, including the creation of debris.
Composite discs comprise of base plates pivotally coupled by a polymer core. Examples of composite discs include DEPUY SYNTHES' “PRODISC-C,” MEDTRONIC'S “BRYAN,” NUVASIVE'S “PCM,” GLOBUS MEDICAL'S “SECURE-C,” and ZIMMER BIOMET'S “MOBI-C.” These products provide some shock absorption but do not cure the deficiencies of friction or wear. Furthermore, both mechanical and composite discs have cores that can separate, increasingly the likelihood of injury. Thus, composite discs in the prior art are susceptible to failure and potential injury.
U.S. Pat. No. 8,679,181 discloses DEPUY SYNTHES' “PRODISC-C” composite disc, which comprises a lower base coupled to a convex inner surface and an upper base coupled to a concave inner surface. The joint formed by the convex and concave surfaces more freely articulates and therefore are more likely to dislocate. Thus, the movement of the joint or bearing surfaces creates stress concentration and causes friction, thereby creating debris.
U.S. Pat. No. 8,092,542 discloses MEDTRONIC'S “BRYAN” composite disc, which comprises a polymer core between two concave base plates. When one side of the disc is compressed, the polymer core is displaced in an opposing direction. However, much friction is caused and the friction between the polymer core and the base plates can cause wear and create debris, which could be hazardous to the human body.
U.S. Pat. No. 8,591,586 discloses NUVASIVE'S “PCM” composite disc, which operates similarly to MEDTRONIC'S “BRYAN” and suffers from the same disadvantages.
U.S. Pat. No. 8,167,948 discloses GLOBUS MEDICAL'S “SECURE-C” composite disc, which allows the polymer core between the two concave base plates to move within a predefined range to allow for translational motion. While allowing for translational motion, it still suffers from the other disadvantages of the prior art mentioned above.
U.S. Pat. No. 8,858,635 discloses ZIMMER BIOMET'S “MOBI-C” composite disc, which features large indentations on the polymer core matched to smaller protrusions on the base plates. The protrusions limit movement of the core to the range allowed for by the indentations. While this might provide a safeguard against separation, it also introduces another point of friction between the core and the protrusion. In addition, the superior base member is still free from the core, which may cause dislocation.
Elastic discs in the prior art usually comprise of a rubber core between the base plates. An example of an elastic disc is disclosed by U.S. Pat. No. 8,377,138 and manufactured by SPINAL KINETICS as “M6-C.” It comprises a compressible core encased in fibrous material. Although this product provides improved shock absorption and mechanical resistance to prevent dislocation during motion, the product is more difficult and costlier to manufacture due to the complexity of the components.
Another artificial disc in the prior art is U.S. Pat. No. 9,084,681, which discloses an intervertebral implant with a core comprising a bag filled with beads. This would provide increased shock absorption but the lack of structural support increases the likelihood of failure.
In the prior art, the base plates, or endplates, are commonly made from titanium alloy or cobalt chromium with a porous finish or porous coating to allow fusing with the natural bone. Protrusions on the base plates provide traction and prevent the disc from sliding.
The cores of the elastic or composite discs are typically made from a radiolucent polymer. This is problematic because it prevents clear x-ray imagery of the core for a surgeon to evaluate intraoperatively whether the implant is properly sized and positioned.
Intervertebral discs can be implanted anteriorly or posteriorly. When implanted posteriorly, access is restricted by surrounding nerves and requires a narrower implant. U.S. Pat. No. 8,864,832 discloses an intervertebral implant for posterior implantation. The implant comprises a ball and socket between two narrow base plates. However, this device exhibits all the disadvantages of similar devices for anterior implantation.
As seen above, typical failure of disc implants include dislocation of joints, device migration, subsidence and wear debris. Typical causes of failure come from the device components becoming loosened and the failure to support the load required especially due to lordosis—the difference between the anterior and posterior height.
Therefore, a need exists for an artificial intervertebral disc that solves the problems in the prior art and that is low-friction, wear-resistant, shock-absorbent, debris-free, and provides stability in all directions that is implantable anteriorly and posteriorly.